The indirect method of blood pressure determination usually involves the detection of the Korotkoff sounds produced in the vicinity of the brachial artery when a Rive-Rocci cuff is inflated about the arm of a patient to a pressure between the systolic and diastolic values. Numerous studies have been made to identify the origin and nature of these sounds in efforts to improve the accuracy and reliability of the Korotkoff method of measuring blood pressure. Although there is still disagreement as to the exact mechanism by which the sounds are produced, the detection and recording of the sounds is a straightforward process, and the frequency spectrum of the sound energy has been well established by several investigators.
It is with the detection of those sounds in the presence of high noise levels that the present invention is concerned. This task is made particularly difficult because of the resonant nature of the cuff-tissue-arterial system, which tends to produce energy in the band of interest whenever the system is perturbed by noise impulses, either through the limb or via the cuff.
Noise impulses are produced by arm motion, external vibration or motion coupled through the cuff, tubing, or in many cases from the hand up through the tissues, bones, and blood stream to the area beneath the cuff. It appears that the brachial artery itself is a very efficient conductor of such artifacts, and this is an important factor contributing to the difficulty of measuring blood pressures during physical activity, such as an exercise tolerance test.
The fact that externally generated artifacts tend to arrive at all points beneath the cuff within a few milliseconds of each other, regardless of their source or coupling device, makes it possible to identify legitimate Korotkoff sounds by their relatively unique time relationships. Korotkoff sounds never appear within 15 milliseconds of each other at two microphones spaced by 5 cm and located over the brachial artery. Noise impulses are always synchronized within a few millseconds.
Tests indicate that the events leading to the production of the Korotkoff sound begin with the arrival of the pressure wave at the upper edge of the cuff and are propagated along the brachial artery to the lower edge of the cuff, where the sound is normally detected. The indicated velocity of propagation of this phenomenon approaches the blood flow velocity when the cuff pressure is near systolic value, and approaches the pulse wave velocity when the cuff pressure is near diastolic. Since these two velocities differ by approximately a 10:1 ratio, and artifact sounds or noise impulses tend to travel at the pulse wave velocity or faster, the Korotkoff sounds exhibit unique characteristics when compared to other sound events, and can be discriminated from other sounds even though they may be in the same frequency band.
Furthermore, the determination of diastolic pressure need not depend upon the preception of the "muffling" or "disappearance" of the sound as the cuff is deflated, but can be clearly and specifically identified as the pressure at which the sounds-if any-appear to be propagated at a velocity of 5 meters per second or greater. This represents a delay of less than 10 milliseconds between sounds detected by two microphones spaced 5 cm apart along the artery.